Photosensors including photodiode control electrodes and methods of operating same

ABSTRACT

A sensor includes a substrate, a floating diffusion node in the substrate, a photodiode in the substrate laterally spaced apart from the floating diffusion region and a transfer transistor coupling the photodiode and the floating diffusion region. The sensor further includes a photodiode control electrode disposed on the photodiode and configured to control a carrier distribution of the photodiode responsive to a control signal applied thereto. The floating diffusion region may have a first conductivity type, the photodiode may include a first semiconductor region of a second conductivity type disposed on a second semiconductor region of the first conductivity type, and the photodiode control electrode may be disposed on the first semiconductor region. The photodiode may be configured to receive incident light from a side of the substrate opposite the photodiode control electrode. The transfer transistor may include a gate electrode on a channel region in the substrate and the photodiode control electrode and the transfer transistor gate electrode may be separately controllable. In further embodiments, the photodiode control electrode comprises an extension of the transfer transistor gate electrode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/132,814, filed Jun. 4, 2008 which claims the benefit of Korean PatentApplication No. 10-2007-0055728, filed Jun. 7, 2007, the disclosures ofwhich are hereby incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to semiconductor photosensors and methodsof operation thereof and, more particularly, to CMOS sensors and methodsof operation thereof.

BACKGROUND OF THE INVENTION

CMOS image sensors are used to transform incident light energy intoelectrical signals, and are widely used in devices such as digitalcameras and video recorders. FIG. 1 is a circuit diagram of a typicalphotocell of a CMOS image sensor. The photocell includes a photodiode PDthat generates carriers responsive to incident light Lin. Referring toFIGS. 1 and 2, in an image capture cycle, carriers are transferred fromthe photodiode PD to a floating diffusion node FDN by a transfertransistor TTr responsive to a signal S_TG applied to a gate TG of thetransfer transistor TTr. A potential developed at the floating diffusionnode FDN is proportional to the amount of transferred change, and isused to drive the gate of an amplifier transistor FTr to control acurrent passing through a select transistor STr. A reset transistor RTris used to reset the floating diffusion node FDN for a succeeding imagecapture cycle.

FIG. 3 is a cross-sectional view of a conventional CMOS photocell. Aphotodiode PD includes an n-type region 20 formed in a p-type substrate10 and a p+-type 30 region formed on the n-type region 20. A transfertransistor 40 that couples the photodiode PD to an n+-type floatingdiffusion node region FDN in the substrate 10 includes a transfer gateTG overlying a channel region CH_T in the substrate 10. A resettransistor 50 that couples the floating diffusion node region FDN to areset node region RN in the substrate 10 includes a reset gate RGoverlying a channel region CH_R in the substrate 10. Light Lin isincident on the photodiode PD from a front side of the substrate 10 inan area F1 overlying the p+-type region 30. FIG. 4 illustrates apotential profile for the device shown in FIG. 3.

SUMMARY OF THE INVENTION

Some embodiments of the present invention provide a sensor including asubstrate, a floating diffusion node in the substrate, a photodiode inthe substrate laterally spaced apart from the floating diffusion regionand a transfer transistor coupling the photodiode and the floatingdiffusion region. The sensor further includes a photodiode controlelectrode disposed on the photodiode and configured to control a carrierdistribution of the photodiode responsive to a control signal appliedthereto. The floating diffusion region may have a first conductivitytype, the photodiode may include a first semiconductor region of asecond conductivity type disposed on a second semiconductor region ofthe first conductivity type, and the photodiode control electrode may bedisposed on the first semiconductor region. The photodiode may beconfigured to receive incident light from a side of the substrateopposite the photodiode control electrode. The transfer transistor mayinclude a gate electrode on a channel region in the substrate and thephotodiode control electrode and the transfer transistor gate electrodemay be separately controllable. In further embodiments, the photodiodecontrol electrode includes an extension of the transfer transistor gateelectrode.

Further embodiments provide a sensor including a photodiode including afirst semiconductor region having a first conductivity type disposed ona second semiconductor region having a second conductivity type and atransfer transistor having a channel coupled to the second semiconductorregion. The sensor also includes a photodiode control electrode disposedon the first semiconductor region opposite the second semiconductorregion. The sensor further includes a control circuit coupled to thephotodiode control electrode and the transfer transistor and configuredto bias the photodiode control electrode to create a majority-carrierconcentration region in first semiconductor region near the photodiodecontrol electrode. In some embodiments, the control circuit may beconfigured to bias the photodiode control electrode to create amajority-carrier concentration region in the first semiconductor regionnear the photodiode control electrode while causing the transfertransistor to impede charge transfer through the channel of the transfertransistor.

In some embodiments, the control circuit may be further configured toallow charge transfer through the transfer transistor while biasing thephotodiode control electrode to increase a potential across thephotodiode. For example, the control circuit may be configured to biasthe photodiode control electrode and a gate electrode of the transfertransistor with voltages of opposite polarities to allow charge transferthrough the transfer transistor while increasing the potential acrossthe photodiode.

In other embodiments, the control circuit may be further configured toallow charge transfer through the transfer transistor while biasing thephotodiode control electrode to increase a minority carrierconcentration in the first semiconductor region near the photodiodecontrol electrode. For example, the control circuit may be configured tobias the photodiode control electrode and a gate electrode of thetransfer transistor to the same voltage.

Still further embodiments of the present invention provide a sensorincluding a photodiode including a first semiconductor region having afirst conductivity type disposed on second semiconductor region having asecond conductivity type, a transfer transistor having a channel coupledto the second semiconductor region and a photodiode control electrodedisposed on the first semiconductor region opposite the secondsemiconductor region. The sensor further includes a control circuitelectrically coupled to the photodiode control electrode and thetransfer transistor and configured to bias the photodiode controlelectrode to increase a potential across the photodiode while enablingcharge transfer through the transfer transistor. The control circuit maybe configured to bias the photodiode control electrode and a gateelectrode of the transfer transistor with voltages of oppositepolarities to allow charge transfer through the transfer transistorwhile increasing the potential across the photodiode.

In additional embodiments of the present invention, a sensor includes aphotodiode including a first semiconductor region having a firstconductivity type disposed on second semiconductor region having asecond conductivity type, a transfer transistor having a channel coupledto the second semiconductor region, and a photodiode control electrodedisposed on the first semiconductor region opposite the secondsemiconductor region. The sensor further includes a control circuitelectrically coupled to the photodiode control electrode and thetransfer transistor and configured to enable charge transfer through thetransfer transistor while biasing the photodiode control electrode toincrease a minority carrier concentration in a portion of the firstsemiconductor region near the photodiode control electrode. The controlcircuit may be configured to bias the photodiode control electrode and agate electrode of the transfer transistor to the same voltage.

Some embodiments of the present invention provide methods of operating asensor including a photodiode and a transfer transistor having a channelcoupled to the photodiode in which a bias is applied to a photodiodecontrol electrode disposed on the photodiode to control a carrierdistribution of the photodiode while controlling the transfer transistorto impede and/or allow charge transfer from the photodiode. Thephotodiode may include a first semiconductor region of a firstconductivity type disposed on a second semiconductor region of a secondconductivity type, the second semiconductor region coupled to thechannel of the transfer transistor. A bias may be applied to thephotodiode control electrode to create a majority-carrier concentrationin the first semiconductor region near the photodiode control electrodewhile impeding charge transfer from the second semiconductor regionthrough the transfer transistor. In further method embodiments, a biasmay be applied to the photodiode control electrode to increase apotential across the photodiode while enabling charge transfer throughthe transfer transistor. In still further embodiments, a bias may beapplied to the photodiode control electrode to increase a minoritycarrier concentration in the first semiconductor region near thephotodiode control electrode while enabling charge transfer through thetransfer transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a conventional CMOS photocell.

FIG. 2 is a waveform diagram illustrating operations of the conventionalCMOS photocell of FIG. 1.

FIG. 3 is a cross-sectional view depicting a conventional CMOSphotocell.

FIG. 4 is a potential diagram illustrating operations of theconventional CMOS photocell of FIG. 3.

FIG. 5 is a cross-sectional view depicting a CMOS photocell according tosome embodiments of the present invention.

FIG. 6 is a waveform diagram illustrating operations of the CMOSphotocell of FIG. 5 according to some embodiments of the presentinvention.

FIG. 7 is a potential diagram illustrating operations of the CMOSphotocell of FIG. 5 according to some embodiments of the presentinvention.

FIG. 8 is a cross-sectional view depicting a CMOS photocell according tofurther embodiments of the present invention.

FIG. 9 is a waveform diagram illustrating operations of the CMOSphotocell of FIG. 8 according to some embodiments of the presentinvention.

FIG. 10 is a potential diagram illustrating operations of the CMOSphotocell of FIG. 8 according to some embodiments of the presentinvention

DETAILED DESCRIPTION

The invention is described more fully hereinafter with reference to theaccompanying drawings, in which embodiments of the invention are shown.This invention may, however, be embodied in many different forms andshould not be construed as limited to the embodiments set forth herein.Rather, these embodiments are provided so that this disclosure will bethorough and complete, and will fully convey the scope of the inventionto those skilled in the art.

It will be understood that when an element or layer is referred to asbeing “on,” “connected to” and/or “coupled to” another element or layer,the element or layer may be directly on, connected and/or coupled to theother element or layer, or intervening elements or layers may bepresent. In contrast, when an element is referred to as being “directlyon,” “directly connected to” and/or “directly coupled to” anotherelement or layer, no intervening elements or layers are present. As usedherein, the term “and/or” includes any and all combinations of one ormore of the associated listed items.

It will also be understood that, although the terms “first,” “second,”etc., may be used herein to describe various elements, components,regions, layers and/or sections, these elements, components, regions,layers and/or sections should not be limited by these terms. Rather,these terms are used merely as a convenience to distinguish one element,component, region, layer and/or section from another element, component,region, layer and/or section. For example, a first element, component,region, layer and/or section could be termed a second element,component, region, layer and/or section without departing from theteachings of the present invention.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” “top,” “bottom” and the like, may be used to describe anelement and/or feature's relationship to another element(s) and/orfeature(s) as, for example, illustrated in the figures. It will beunderstood that the spatially relative terms are intended to encompassdifferent orientations of the device in use and/or operation in additionto the orientation depicted in the figures. For example, when the devicein the figures is turned over, elements described as below and/orbeneath other elements or features would then be oriented above theother elements or features. The device may be otherwise oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein interpreted accordingly. As used herein,“height” refers to a direction that is generally orthogonal to the facesof a substrate.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to limit of the invention. As usedherein, the singular terms “a,” “an” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprise,”“comprising,” “includes,” “including,” “have”, “having” and variantsthereof specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence and/or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

Embodiments of the present invention may be described with reference tocross-sectional illustrations, which are schematic illustrations ofidealized embodiments of the present invention. As such, variations fromthe shapes of the illustrations, as a result, for example, ofmanufacturing techniques and/or tolerances, are to be expected. Thus,embodiments of the present invention should not be construed as limitedto the particular shapes of regions illustrated herein, but are toinclude deviations in shapes that result from, e.g., manufacturing. Forexample, a region illustrated as a rectangle may have rounded or curvedfeatures. Thus, the regions illustrated in the figures are schematic innature and are not intended to limit the scope of the present invention.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of this specification andthe relevant art and will not be interpreted in an idealized or overlyformal sense unless expressly so defined herein.

FIG. 5 illustrates a CMOS photocell 500 according to some embodiments ofthe present invention. The photocell 500 includes a photodiode PD formedin a p-type substrate 510. The photodiode PD includes a p-type region522 disposed on an n-type region 524. A portion of the n-type region 524of the photodiode PD contacts a channel region CH_T of a transfertransistor 530. The channel region CH_T is controlled by a transfer gateelectrode TG disposed thereon on a front side of the substrate 510. Thechannel region CH_T also contacts a floating node that receives chargefrom the photodiode PD, here shown as an n-type floating diffusion nodeFDN disposed in the substrate 510. The photocell 500 further includes aphotodiode control electrode PG disposed on the photodiode PD, adjacentthe transfer gate electrode TG. The photodiode PD is configured toreceive incident light Lin from a backside of the substrate 510. Acontrol circuit 540 is configured to separately control the photodiodecontrol electrode PG and the transfer gate electrode TG, e.g., to applyvoltages S_TG and S_PG thereto. Conceptually illustrated in FIG. 5, itwill be understood that the control circuit 540 may be implemented inany of a number of different ways. For example, the control circuit 540may comprise circuitry located in and/or on the substrate 510 and/orexternal thereto.

Referring to FIGS. 5-7, use of the photodiode control electrode PG mayallow the photocell 500 to provide improved performance overconventional photocells. In particular, as shown in FIG. 6, during aportion of an image capture cycle in which the photodiode isaccumulating carriers responsive to the incident light Lin, the controlcircuit 540 may bias the photodiode control electrode voltage S_PG to afirst negative voltage NEG1 below a ground voltage GND while holding thetransfer gate electrode voltage S_TG at the ground voltage GND. In thisstate, the transfer gate electrode TG impedes charge transfer from thephotodiode PD to the floating diffusion node FDN through the channelCH_T of the transfer transistor while the negative voltage S_PG on thephotodiode control electrode PG causes a majority carrier concentrationto form in a portion of the p-type region 522 of the photodiode PD nearthe photodiode control electrode PG. Formation of this majority-carrierconcentration region may help to reduce dark current of the photodiodePD. As further illustrated in FIG. 6, in a subsequent charge transferphase, the control circuit 540 may drive the transfer gate electrodevoltage S_TG to a higher voltage level sufficient to turn on thetransfer gate transistor 530 while concurrently driving the photodiodecontrol gate voltage S_PG to a more negative voltage NEG2.

Driving the photodiode control electrode PG in this manner during chargetransfer may increase the potential of the photodiode PD, as shown inFIG. 7. In particular, FIG. 7 illustrates a potential profile Po1 of aconventional photocell along the lines described above with reference toFIGS. 3 and 4 in comparison with a profile Po2 of a photocell accordingto FIG. 6 operated along the lines described above. As can be seen,driving the photodiode control electrode PG with a negative voltage asdescribed above may increase the potential Po2 in comparison to thepotential Pot, which may help accumulated carriers to overcome apotential barrier that may impede transfer between the photodiode PD andthe floating diffusion node FDN.

According to further embodiments of the present invention, improved darkcurrent suppression and/or charge transfer efficiency may be achievedutilizing a photodiode control electrode that is coupled to the gateelectrode of a transfer transistor. FIG. 8 illustrates a CMOS photocell800 according to some embodiments of the present invention. Thephotocell 800 includes a photodiode PD formed in a p-type substrate 810.The photodiode PD includes a p-type region 822 disposed on an n-typeregion 824. A portion of the n-type region 824 contacts a channel regionCH_T of a transfer transistor 830. The channel region CH_T also contactsan n-type floating diffusion node FDN disposed in the substrate 810. Thephotocell 800 further includes an extended electrode TGe that includes afirst portion overlying the channel region CH_T that serves as a gateelectrode of the transfer transistor 830 and a second portion disposedon the photodiode PD that serves as a photodiode control electrode. Thephotodiode PD is configured to receive incident light Lin from abackside of the substrate 810. A control circuit 840 is configured tocontrol the extended electrode TGe, e.g., to apply a voltage S_TGethereto. It will be appreciated that the control circuit 840 may beimplemented in a number of different ways, for example, using circuitrylocated in and/or on the substrate 810 and/or external thereto.

Referring to FIGS. 8-10, the extended electrode TGe may allow thephotocell 800 to provide improved performance over conventionalphotocells. In particular, as shown in FIG. 9, during a portion of animage capture cycle in which the photodiode PD is accumulating carriersresponsive to the incident light Lin, the control circuit 840 may biasthe extended electrode voltage S_TGe to a negative voltage NEG below aground voltage GND. In this state, charge transfer through the channelCH_T of the transfer transistor 830 is impeded while the negative biasof the portion of the extended electrode TGe overlying the photodiode PDcauses a majority carrier concentration region to form in the p-typeregion 822 of the photodiode PD near the extended electrode TGe.Formation of this majority-carrier concentration region may help toreduce dark current through the photodiode PD.

As further illustrated in FIG. 9, in a subsequent charge transfer phase,the control circuit 840 may drive the extended electrode voltage S_TGeto a higher voltage sufficient to turn on the transfer transistor 830.Driving the extended electrode TGe in this manner during charge transfermay lower a potential barrier near the transfer transistor channel CH_Tby increasing a minority carrier concentration in a portion of thep-type region 822 of the photodiode PD near the extended electrode TGe,as shown in FIG. 7. In particular, FIG. 10 illustrates a potentialprofile Po1 of a conventional photocell along the lines described abovewith reference to FIGS. 3 and 4 with the channel CH_T of a transfertransistor turned on, in comparison with a profile Po2 of a photocellalong the lines of FIGS. 8 and 9 as described above. As can be seen,driving the extended electrode with a negative voltage as describedabove may reduce the potential Po2 with respect to the potential Po1,which may improve efficiency of charge transfer between the photodiodePD and the floating diffusion node FDN.

It will be understood that variations of the structures and operationsdescribed above with reference to FIGS. 5-10 fall within the scope ofthe present invention. For example, while the structures illustratedutilize photodiodes and floating nodes formed using doped regions withina substrate, photocells with similar characteristics may be formed usingother structures, e.g., structures that utilize deposited regionsinstead of diffusion regions formed in a substrate. In otherembodiments, a photodiode control electrode structure along the linesillustrated in FIG. 5 may be controlled to provide operations similar tothose described above with reference to FIGS. 8-10, e.g., the photodiodecontrol electrode PG and the transfer gate electrode TG may be drivenwith the same voltages along the lines shown in FIG. 9.

The foregoing is illustrative of the present invention and is not to beconstrued as limiting thereof. Although a few exemplary embodiments ofthis invention have been described, those skilled in the art willreadily appreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe claims. Therefore, it is to be understood that the foregoing isillustrative of the present invention and is not to be construed aslimited to the specific embodiments disclosed, and that modifications tothe disclosed embodiments, as well as other embodiments, are intended tobe included within the scope of the appended claims.

1. A sensor comprising: a substrate; a floating diffusion node in thesubstrate; a photodiode in the substrate laterally spaced apart from thefloating diffusion region; a transfer transistor coupling the photodiodeand the floating diffusion region; and a electrode disposed on thephotodiode to apply a control signal, wherein the photodiode isconfigured to receive incident light from a side of the substrateopposite the photodiode control electrode.